The first set with two breaks won Halep, but in the second game the scenario was already written Svitolina Elina took two games at the reception without problems and transferred the semi-finals in the deciding set.
The Chan sisters took just 61 minutes to complete a 6-3, 6-2 victory over Lara Arruabarrena of Spain and Kaitlyn Christian of the United States in their quarter-final at the Khalifa International Tennis and Squash Complex.
Mertens, 23, recovered from a set and a break down - losing 18 points in a row at one stage in the second set - to beat the world number three 3-6 6-4 6-3.
Earlier, Kei Nishikori continued his unbeaten debut week with a 6-3, 6-2 defeat of Hungary's Marton Fucsovics on Friday to reach the semi-finals on the back of a performance he described as "almost perfect".
"Simona is a great player and it's nice to get the trophy", the Belgian said.
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"She moved very well and she hit the ball strong".
"I couldn't say I was really nervous", she added, "but I think my shots went long".
"I wanted to lift this trophy badly but I couldn't and Elise deserves it very much", said Halep.
Mertens' victory was the second time this week she has beaten a player in the top 10, having dispatched world No. 8 Kiki Bertens in the quarterfinals.
Third seed Angelique Kerber battled back from a set down to oust Barbora Strycova. Kerber had a poor opening set but she managed to show her class in the 2nd and 3rd sets to secure the overall victory.
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Angelique Kerber and Elise Mertens have played once in the past.
The unseeded Belgian saved 11 of the 17 break points she faced and won against serve three times in the decider to seal her first WTA title of the year, while Halep's wait for a taste of silverware in 2019 continues. "I was trying to play better than I did in the first set".
The Romanian broke back but was soon struggling again and Mertens was crowned champion when her opponent drilled a double-handed backhand into the net. I felt a little bit pressure that I'm going to lose the match. "It was a really close match and I think just one, two points decide the match".
She said it outshone her achievement in getting to the Australian Open semi-finals in 2018.
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